1. Field of the Invention
The present invention relates to the field of couplers having the function of sampling part of the power present on a main line towards a secondary line for control and feedback. Couplers are generally used in closed-loop gain-control systems to provide a real measurement of the power.
2. Discussion of the Related Art
FIG. 1 very schematically shows an example of a conventional circuit using a coupler. This example relates to the control of a power amplifier 1 (PA) for amplifying a useful signal UTI for a transmit antenna 2. In this type of application, the transmit power is controlled with a power reference PL. A coupler 3 is interposed between the output of amplifier 1 and antenna 2 to extract data proportional to the power actually transmitted. This data is exploited by a detector 4 (DET) providing a measured value MES to a comparator 5 with required power PL. Comparator 5 provides a control signal REG to amplifier 1.
Two large coupler categories are essentially known. A first category relates to so-called distributed couplers, which are formed from coupled transmission lines. A second category relates to couplers with local components, based on capacitors and inductances.
Distributed couplers are directional, that is, they detect the direction of the measured power and are sensitive to dimensional variations of the lines. Such couplers are bulky due to the size of the lines to be formed, especially for radio frequency applications (from several hundreds of MHz to a few GHz).
Couplers with local components are non-directional. They have the advantage of having a large passband and of being more compact.
As illustrated in FIG. 1, a coupler is defined by four ports or terminals IN, DIR, CPLD, and ISO. Terminals IN and DIR are on the main line while terminals CPLD and ISO define the coupled secondary line. In FIG. 1, terminal IN is on the side of power amplifier 1 while terminal DIR is on the side of antenna 2. Terminal CPLD is the terminal on which is sampled the information proportional to the power in the main line. In a non-directional coupler, to which the present invention applies, terminals IN and DIR are one and the same and terminal ISO generally does not exist.
The main parameters of a non-directional coupler are:
the coupling factor (generally on the order of from 10 to 30 dB) which corresponds to the path loss between ports IN and CPLD (the other port being loaded with a standardized impedance, generally 50 ohms); and
the insertion loss in the desired passband which corresponds to the path loss between ports IN and DIR (the other port being loaded with a standardized impedance, generally 50 ohms) and which is desired to be as small as possible (smaller than 1 dB and preferably on the order of 0.5 dB) to minimize the attenuation of the wanted signal due to the presence of the coupler.
FIG. 2 shows the electric diagram of a conventional non-directional coupler with local components. Such a coupler is essentially formed of the association of two cells 31 and 32 respectively forming high-pass and low-pass filters. Cell 31 comprises a capacitor C31 having a first electrode connected to transmit line 12 (confounded terminals IN and DIR) and having a second electrode connected, by an inductance L31, to ground. The second electrode of capacitor C31 also constitutes an input terminal of cell 32 formed of an inductance L32 connecting this second electrode to terminal CPLD, terminal CPLD being further grounded by a capacitor C32.
A disadvantage of passive couplers with local components such as that illustrated in FIG. 2 is linked to the dispersions (on the order of 20%) of the inductive and capacitive components upon manufacturing thereof. Such dispersions are reflected on the coupler parameters, which are given for an operating frequency band.
Theoretically, it is also possible to form high-pass and low-pass filters based on resistive and capacitive elements to form a coupler. However, the required number of stages (filter order) results, in practice, in a large size filter. Further, the dispersion problem is also present for resistors.
Above all, such structures are, in practice, not integrable in high-frequency applications (over one hundred MHz) more specifically aimed at by the present invention, due to the small required values, especially for capacitors (less than one picofarad).